Exemplary embodiments of the present invention relate generally to soot sensors and methods for sensing soot.
Soot sensors, used to measure particulate in, e.g., engine exhaust, typically can be established by simple resistive devices. Such a sensor typically consists of a non-conductive substrate, most often alumina, with a screen print pattern using a conductive material, often a precious metal in order to withstand the temperature of a co-fire (although co-fire may not be necessary). In some instances the sensor will also have a heater to heat the soot sensor as required.
The soot sensing portion of the element will have two electrodes with inter-digitized “fingers” that maximizes a perimeter between the two electrodes. When soot from the exhaust lands on the sensor, the carbon makes a high resistance short between the electrodes, effectively lowering the resistance. The more the soot collects, the lower the resistance, and this resistance is measured as an indication of the amount of soot. If it is desired to clean off the soot from the element, the heater on the element is activated to clean off the element. It is also desirable to shorten the distance between the electrodes thus increasing the sensitivity of the signal of the sensing element.
One of the primary issues with soot sensing is that the soot tends to travel by “line of sight”, which means that if the soot detecting electrodes are not in the direct line of sight of the soot particles (e.g., rotated by 90 degrees) then the soot will not deposit on the electrodes and generate a signal. Accordingly, there is a need for an orientation insensitive element since it is difficult to orientate the sensor to the exhaust stream. Furthermore and in order to make the soot sensing element more sensitive it is desirable to reduce the spacing between the electrodes and provide more electrode surface area.
Accordingly, is desirable to improve the sensitivity of the soot sensor.